In many business applications, color documents have become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Companies involved in the development of color output devices continue to look for ways to improve the total image quality of such devices. One of the elements that affects the perception of image quality is an ability to consistently produce the same quality image output on a printer from one day to another, from one week to the next, month after month. Users are accustomed to printers and copiers that produce high quality color and grayscale output. Users further expect to be able to reproduce a color image with consistent quality on any compatible output device, including another device within an organization, a device at home or a device used anywhere else in the world. Hence, there remains a commercial need for efficiently maintaining print color predictability, particularly as electronic marketing has placed more importance on the accurate representation of merchandise in illustrative print or display media.
Description of color, color perception and psychological and physiological phenomena involving light, object and observer, including color measurements using spectrophotometers are described in R. W. G. Hunt, “The Reproduction of Color in Photography, Printing and Television”. Fourth Edition, Fountain Press, Tolworth, England 1987 ISBN 0-8524-2356.
It will be appreciated that, in an output image for a color rendering device (e.g., printer, copier or other image output device), good quality process gray provides a strong indication of how well a color rendition process is functioning, and how a printer is operating. Gray balance calibration methodologies are: known as indicated in commonly assigned US Patent Publication 2005/0071104, by Viturro et al. for a METHOD FOR CALIBRATING A MARKING SYSTEM TO MAINTAIN COLOR OUTPUT CONSISTENCY ACROSS MULTIPLE PRINTERS, and pending U.S. application Ser. No. 09/566,291 by L. K. Mestha, for an ON-LINE CALIBRATION SYSTEM FOR A DYNAMICALLY VARYING COLOR MARKING DEVICE, filed on May 5, 2000, both applications being hereby incorporated by reference in their entirety, and they enable printing good quality process gray for single, multiple and tightly integrated parallel printers (TIPP).
Recently filed patent applications, (a) U.S. application Ser. No. 10/833,231 for a FULL WIDTH ARRAY SCANNING SPECTROPHOTOMETER by L. K. Mestha et al., filed Apr. 27, 2004, and (b) U.S. application Ser. No. 11/016,952 for a FULL WIDTH ARRAY MECHANICALLY TUNABLE SPECTROPHOTOMETER, by L. K. Mestha, filed Dec. 20, 2004, cover the use of two different types of full-width array (FWA) spectrophotometers to sense a full page. Both co-pending applications are also hereby incorporated by reference for their teachings. Similarly, U.S. application Ser. No. 10/248,387 for SYSTEMS AND METHODS FOR OBTAINING A SPATIAL COLOR PROFILE, AND CALIBRATING A MARKING SYSTEM, by L. K. Mestha et al., filed on Jan. 15, 2003, and also hereby incorporated by reference, shows an approach that utilizes a specially designed test pattern containing numerous patches, some of which are arranged side by side along the slow scan direction (y-direction) to obtain the gray tonal reproduction curves (TRCs). The remaining patches are organized along the fast scan direction (x-direction) to obtain spatial gray balance TRCs for spatial uniformity correction. A patch target is printed and measured using an in-line or off-line spectrophotometer for obtaining such a gray balance map.
However, such calibration systems and methods do not correct for uniformity defects at a wide range of spatial frequencies, like streaks, bands, “smile/frown”, etc. In principle, performing gray balance calibration using full-page information enables the correction of all such defects while maintaining the best uniform process gray a marking engine can produce. Accordingly, the disclosed system and methods utilize a hybrid sensing system to achieve similar results in a readily implemented manner. It will be further appreciated that the hybrid sensing system may be implemented with localized or less than full-width sensing devices, and as such enables the performance of the disclosed calibration process without significant additional cost.
More specifically, the following disclosure is directed to a sensing system comprising a spectrophotometer (e.g., Low Cost Light Emitting Diode (LCLED) spectrophotometer) and the use of uniformity measurements from a full-width array (FWA) scanner bar to measure developability non-uniformity on a photoreceptor or transfer belt. Such devices, in combination, are suitable to provide a printer or similar output device with streak detection and correction capabilities. The spatial mapping algorithms used for this approach form an aspect of this disclosure, and they provide spatial color maps (gray balance TRCs) to achieve color consistency in single printers, in color TIPP configurations, and even among multiple printers.
Obtaining TRCs for a particular color output device or marking engine is a calibration process, which can be constructed by printing predetermined target colors and measuring the printed target colors using in situ color sensors. Predetermined target colors can be printed as chronological jobs in the banner sheet/header sheet and measured either by measuring straight from the output image or by rendering subsets of customer colors as target color patches in banner or header pages. For example, U.S. Pat. No. 6,538,770, issued Mar. 25, 2003 is directed to a control system using dual-mode, banner color test sheets, and is hereby incorporated by reference. Using the target colors and their measured counterparts, and by processing the measured colors, TRCs are adjusted on-line at some desired intervals or on request during system or color balance set ups. Generally, obtaining one-dimensional TRCs is associated with achieving neutral gray balance, where the “grayness” of a color is an indication of how “clean” a process color is, compared to its theoretical ideal. Good gray can be characterized as having zero chroma (that is, a*=0=b*on an L*a*b*scale).
When equal amounts of cyan, magenta and yellow marking materials are printed on a white paper, a well balanced printer should produce a neutral gray of the same amount. Often, however, a brownish color is observed rather than a neutral gray. The system will not produce the desired gray due to various limitations on color pigments of the primaries and the internal processes of the print engine. To overcome this effect, gray balanced TRCs are used as one-dimensional look-up tables (LUTs) to modulate the amount of cyan, magenta and yellow proportions depending on the state of the materials and the print engine. The TRCs are obtained by printing large number of patches, mostly near neutral. In the methods practiced by the color reproduction industry, colors are measured using offline spectrophotometers and measured quantities are then modified, generally, by using model based algorithms to produce the desired gray balanced TRCs. Usually this process of printing and producing TRCs is iterated several times until satisfactory results are obtained. This type of approach is time consuming and expensive due to the use of machine models and offline spectrophotometer hardware.
The embodiments disclosed herein include a color printer gray balance method that employs a test target with a small number of patches—that can be made automatic and reliable—to assure color consistency between calibrations for single and multiple machines. Using the information provided by a FWA scanner bar on a spatial area determined by the spatial measurement resolution of the scanner bar on the photoreceptor belt or in the printed sheet the method develops a gray balance map across scan and process direction at the same or higher resolution. The disclosed system and method build on the prior applications referenced above to achieve color consistency using closed feedback loop controls and in-line (or off-line) spectrophotometers for single, multiple and TIPP color printer configurations, and more particularly spatial mapping algorithms and procedures to obtain spatial color maps that can achieve linearization to a gray “axis” for an entire page. This approach can also improve page uniformity by providing compensation for detected streaks and bands.
Accordingly, a method is implemented on a printing or similar output device to spatially gray balance a color printer or similar output device. The method and system employ a hybrid sensing system that combines spectrophotometer sensing with a full page image scanner of FWA. The spectral color is measured by a spectrophotometer, while the two-dimensional reflectance is measured by an image scanner. The latter could be sensed from an image on paper, or possibly toned images on an image belt or drum (e.g., photoreceptor or transfer belt). The methods and associated algorithms produce gray balance tone reproduction curves (TRC) at each spatial location and produce Black TRC for each spatial location.
Disclosed in embodiments herein is a method for spatial gray balance calibration of a color output device, comprising: producing an output image with the device in response to an input signal from a test image, wherein the test image includes at least one preselected color; measuring with a first sensor the image corresponding to the preselected color, said first sensor producing a first output indicating spectral color information for at least the preselected color; producing gray balance TRCs using measurements from at least one preselected color; producing another output image with the device in response to a request, wherein the test image includes at least one preselected color located at a plurality of spatial locations in the test image; measuring with a second sensor the image at preselected spatial location(s) corresponding to the preselected color, said second sensor producing a second output indicating reflectance information for at least the preselected color at the plurality of spatial locations; determining an error between the measured color of the one preselected color at a preselected pixel location (or an average of a block of pixels in the preselected spatial region of the preselected color) and using the color information and the reflectance information at other pixel locations (or blocks containing multiple pixels) adjusting the gray balance tone reproduction curves (TRCs) of the device to minimize the spatial uniformity errors at all pixel locations, thus calibrating the device color output spatially, whereby the device expeditiously produces pleasing uniform color.
Also disclosed in embodiments herein is a color output device including a calibration system for spatial gray balance of an output image, the system comprising: a front end converter for converting an input signal representative of a target image comprising a preselected color into a device-dependent control signal in accordance with a device TRC; a color marking engine, responsive to the device-dependent control signal, for generating a marked image in response thereto at a plurality of spatial locations; a first sensor for measuring the marked image corresponding to the preselected color, said first sensor producing a first output indicating spectral color information for at least the preselected color; a second sensor for measuring the marked image at preselected spatial location(s) corresponding to the preselected color, said second sensor producing a second output indicating reflectance information for at least the preselected color; and a controller for producing gray balance TRCs using measurements from at least one preselected color, producing another output image with the device, wherein the test image includes at least one preselected color, and determining an error between the measured color of the one preselected color at a preselected pixel location (or an average of a block of pixels in the preselected spatial region of the preselected color) and using the color information at other pixel locations (or blocks containing multiple pixels) constructing the gray balance TRCs for the device to minimize spatial uniformity errors at all pixel locations, thus calibrating the device color output spatially, whereby the device expeditiously produces pleasing uniform color.
Another disclosed feature of the embodiments herein is a color output device, comprising: a source of printable substrates, said source operatively connected to a printing engine, said printing engine producing an output image with the device in response to an input signal from a test image, wherein the test image includes at least one preselected color; a spectrophotometer, said spectrophotometer measuring the image corresponding to the preselected color, said spectrophotometer producing a first output indicating spectral color information for at least the preselected color; a controller for producing gray balance tone reproduction curves using measurements from at least one preselected color, and producing a second output image with the device in response to a request, wherein the second output image includes at least one preselected color located at a plurality of spatial locations in the test image; and a reflectance scanner for sensing a developed color region of the second output image at a preselected spatial location corresponding to the preselected color, said reflectance scanner producing a second output indicating reflectance information for at least the preselected color at the plurality of spatial locations; said controller determining an error between the measured color of the preselected color at a preselected pixel location and, using the color information and the reflectance information at other pixel locations, adjusting the gray balance tone reproduction curves of the device to minimize the spatial uniformity errors at all pixel locations, thereby calibrating the device color output spatially.